1. Field of the Invention
The present invention relates to a gradation conversion method for video signal processing, and is particularly suitable for adjusting the brightness of a video input signal in a video camera or video hard copy printer.
2. Description of the Prior Art
With recent advances in hard copy printing technologies, and particularly hard copy color printing technologies, it has become possible in the last few years to produce hard copy printouts of video and computer images with high-fidelity reproduction of source image colors using subliminal heat transfer printing and other printing technologies. By using new recording materials and image processing techniques, including high definition television technologies, the resolution and color reproducibility of these hard copy printouts now approach that of conventional silver halide photographic media.
Even the best printers, however, have a limited dynamic color range that compares unfavorably with the dynamic range of a CRT by a factor of ten or more. It is therefore possible to obtain color printouts with color characteristics comparable to those of the CRT only when the dynamic range of the video input signal does not exceed the dynamic range of the printer, and the dynamic range is used to the maximum. These limitations have led to the development of specific automatic gain control and black level correction methods (Japanese Laid-Open Patent Publication No. S62-209671) to improve print quality.
When adjusting image brightness in a television receiver, a luminance signal offset value is normally provided to control the image brightness by shifting the luminance signal level. This method is effective when used with a CRT or other display device having a wide dynamic range, but is not as effective when applied in a printer. This is because the dynamic range of the image normally exceeds the dynamic range of the printer. Any shift in the brightness level is thus limited by the maximum ink density (when adjusting for a darker image) and the maximum brightness of the print medium (when adjusting for a lighter image). Brightness adjustment therefore causes a loss of image information, and results in reduced image quality.
A different method, which changes the gradation characteristics of the image without changing the dynamic range of the image, is therefore used. Specifically, the gradation characteristics are adjusted by controlling the brightness histogram of the image and achieve an effective visual change in image brightness.
As shown in FIG. 11(A), this is achieved by changing the gradation characteristics using gamma conversion of the RGB or luminance signals. To make the image appear brighter, the brightest and darkest parts of the image are left unchanged while the intermediate levels are adjusted along a smooth curve with enhanced brightness. The histogram of image brightness is thus shifted to an increased brightness level without changing the dynamic range of the image, and the image appears "brighter" to the viewer.
Another application of this technology is backlighting compensation in a video camera.
When a subject is recorded against a bright background by a video camera, the lens iris is opened more than usual to increase amount of light from the subject. As shown in FIG. 11 (B), this results in a saturated, white background with no brightness gradations (tones). Gamma conversion is also an effective means for achieving brighter intermediate tones by changing the gradation characteristics when recording under strong backlighting conditions.
These conventional gradation correction methods function effectively with monochrome (gray scale) images, but the hue and chromaticity (saturation) are also changed when these methods are used to adjust the brightness in color images.
FIG. 12 is a graph of the output signals obtained when conventional gamma conversion gradation correction is applied to the RGB signals. In this example the RGB signal values of the input signal are, respectively, 0.3, 0.4, and 0.5. In FIG. 12 (A) the image is made brighter. Each of the RGB gradations is converted, the values are greater than those of the input signal, and the image appears brighter. However, the R:G:B ratio of the output signal is different from that of the input signal. The hue and saturation are therefore changed, and the colors are not high-fidelity reproductions of the input image colors. There is a particular drop in saturation because the ratio is now closer to 1:1:1.
A similar result is obtained when the gradation is changed to reduce the RGB signal values for a darker image as shown in FIG. 12 (B). The R:G:B ratio of the input and output signals is again different with the increased ratio resulting in a pronounced, unnatural increase in saturation.
As described above, the problem with this conventional method of adjusting color image brightness by changing the gradation characteristics and brightness histogram of the image is that the hue and saturation of the image are also changed.
FIG. 13 is a graph of gradation correction applied to the luminance signal to brighten the image when the video signal comprises a luminance signal and color difference signals input.
As in the above example, the input signal values are R=0.3, G=0.4, and B=0.5. In the NTSC format, the luminance signal is thus Y=0.3R+0.59G+0.11B=0.381, and the color difference signals are R-Y=-0.081 and B-Y=0.119.
The brightness of intermediate tones is, of course, changed because the gradation characteristics of the luminance signal are changed with this method, but the hue and saturation are also changed in the same way as the gradation characteristics of the separate RGB signals are changed. To obtain a brighter corrected image, the value of the converted luminance signal is greater than that of the input signal, and the amplitude of the color difference signal is left unchanged. Therefore, the reconverted RGB signal component values are R=0.404, G=0.504, and B=0.604, the R:G:B signal ratio is different from that of the source image and approaches 1:1:1, and the hue again changes and the saturation drops.
Another method makes it possible to keep the color signal ratio constant, but when gradation correction is applied to a pixel with a high luminance signal value and the R:G:B ratio is kept constant, the image reproduction range (e.g., R, G, B.ltoreq.1) is exceeded, and the hue and saturation of the reproduced image again become unnatural.
U.S. patent application Ser. No. 07/772,559 of Haruo YAMASHITA et al. filed Oct. 7, 1991, entitled "Gradation Correction Method and Device" and assigned to the same assignee as the present application relates to the present invention.
As described above, the problem with each of the above methods, i.e., gradation correction of the separate R, G, B signals, gradation conversion of the luminance signal, and color signal conversion of high luminance pixels while maintaining a constant color signal ratio, is that it is not possible to only adjust the brightness without also changing image colors.